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Two malicious software programs that help each other stay on computers are proving difficult to remove.

The programs work together by alternately downloading slighter different variations of the other in an attempt to evade antivirus software, wrote Hyun Choi of Microsoft's Malware Protection Center, on Sunday.

One of the malware programs, called Vobfus, was detected in September 2009. It is known as a downloader, or a program that downloads other pieces of code.

Once Vobfus infects a computer, it downloads from a remote command-and-control server a program called Beebone, which is another kind of downloader that installs other malicious programs on a computer. The two work together, downloading variants of the other that are not immediately detected by antivirus products, Choi wrote.

"This cyclical relationship between Beebone and Vobfus downloading each other is the reason why Vobfus may seem so resilient to antivirus products," Choi wrote. Updated antivirus products may detect one variant present on the system; however, newer downloaded variants may not be detected immediately."

Other malware programs have been known to update themselves once a computer is infected. But if the malware is detected and removed, the targeted computer would have to be infected again by an attacker. The approach of Vobfus and Beebone makes it more likely the computer will remain infected.

Vobfus is also a worm that copies itself to removable drives. It uses the autorun function that, if enabled on a computer, causes Vobfus to automatically run and infect Windows computers.

"In the wild, we have observed that Vobfus maintains a very successful removable-drive infection rate," Choi wrote.